This invention relates to a hydraulic control system, and more particularly to such a system having a plurality of two-position, "latching" actuator valves, such as fuel and/or air valves, that are positionally controlled by a pair of torque motors and associated hydraulic valves, together with a single hydraulic multiplexing selector valve.
In the art of hydraulic control systems or networks for apparatus such as aircraft, and in particular, an aircraft jet engine, the trend is towards more complex systems. The increased complexity is due to a desire to improve fuel efficiency. However, the increased complexity has provoked corresponding undesirable increases in the weight and cost of these control systems, along with a corresponding decrease in their reliability.
Part of the increased complexity stems from an increase in the number of mechanical variables or functions on a jet aircraft engine that must be controlled, either hydraulically or pneumatically. The mechanical variables, such as the engine bleed, typically are controlled by hydraulic or pneumatic actuators that, in turn, are controlled by fuel or air actuator valves.
In the prior art, it is known to provide a control system having actuators controlled by corresponding electromechanical devices, such as torque motors and associated actuator valves. Each torque motor and actuator valve combination converts an electrical input signal from an electronic control into a corresponding mechanical position. With a hydraulic or pneumatic actuator connected to the actuator valve, the electrical signal is converted into a corresponding actuator position.
The primary problem with having a single torque motor dedicated to only a single hydraulic actuator valve and actuator is that, as the number of hydraulic actuators required on the jet aircraft engine increases, the corresponding number of torque motors and actuator valves also increases. This adds to the cost and weight of the overall control system.
Therefore, there are known attempts in the prior art to dedicate a plurality of hydraulic or pneumatic actuator valves and corresponding actuators to a single torque motor. This form of multiplexing is the subject of numerous and varied schemes in the prior art. A common requirement of any multiplexed system is that sufficient hydraulic fluid flow must be provided to each actuator valve. This requirement has caused some multiplexed systems to be generally unreliable due to the lack of the multiplexer, e.g., a pilot valve or a spool valve, to provide adequate flow rate to the actuator valves. The problem is compounded by the usage of actuator valves whose positions vary across the entire range of motion of the movable element of the valve, such as the spool. This "analog" type of continuously modulating actuator valve requires virtually constant high pressure hydraulic fluid to be applied to both sides of the spool to either translate the spool or hold it in a constant position.
Several multiplexer schemes are known in the prior art. One involves a single pilot valve multiplexed among a plurality of actuator valves and associated actuators. The spool of the pilot valve is rotated for multiplexing among a plurality of output ports arranged at different angular positions of the spool. However, this scheme suffers from reduced flow rate to the actuator valves.
Besides rotary-type multiplexer valves, it is known to provide a multiplexer valve whose spool translates linearly to connect an input to one of a plurality of outputs. An example of such a system is given in U.S. Pat. No. 5,048,394, wherein a single torque motor and valve combination controls a hydraulic valve that acts as a multiplexer. The multiplexer connects a single input among a plurality of outputs. The outputs are continuously-modulating actuator valves of the analog-type described hereinbefore. Thus, these valves must be periodically updated, even to merely hold them in their last position, by applying hydraulic fluid to the valve spool. The '394 patent does this constant updating in a synchronous manner by the addition of an oscillator valve to the overall control system. The oscillator valve drives the spool of the multiplexer hydraulic valve linearly back and forth.
Problems with this type of system include the fact that the constant need to update the position of all of the actuator valves can potentially cause torque motor fatigue failures, thereby reducing the lifespan of the torque motor, or constant movement of the oscillator valve could cause valve wear. Also, in order to keep the actuator valves properly positioned, the timing and response of the multiplexer hydraulic valve are critical and, therefore, must be carefully controlled. In addition, and perhaps most importantly, there is no means provided for controlling the position of the actuator valves and actuators if there is a failure of the control system for the torque motors, the control system being typically electronic in nature. If one or more actuator valves and actuators are dedicated to controlling the engine bleed for an aircraft jet engine, the lack of a "fail-safe" condition for the hydraulic/pneumatic actuator valves could create a potentially hazardous condition.
Other examples of prior art multiplexer control systems are given in U.S. Pats. Nos. 4,913,032 and 5,088,383.
Accordingly, it is a primary object of the present invention to provide a multiplexed hydraulic control system having a plurality of two-position, latching actuator valves, such as fuel/air valves, that are controlled by a pair of torque motors and a single multiplexing selector valve.
It is a general object of the present invention to provide such a multiplexed hydraulic control system that overcomes prior art systems that required a dedicated torque motor for each actuator valve.
It is another object of the present invention to provide such a multiplexed hydraulic control system that reduces overall control system weight and cost while also improving reliability.
It is yet another object of the present invention to provide such a multiplexed hydraulic control system that utilizes two-position, "latching" hydraulic actuator valves that do not have to be continuously updated with hydraulic fluid to maintain their position, thereby allowing for a simpler asynchronous control architecture that prolongs the anticipated life of the system components, including the torque motors, and eliminates critical hydraulic valve timing and response requirements found in prior art systems.
It is yet another object of the present invention to provide a multiplexed hydraulic control system that allows the position of each actuator valve to be controlled to a "fail-safe" position in the event that the electronic control system fails.
It is still another object of the present invention to provide a multiplexed hydraulic control system that allows the actuator valves to be activated or translated in any desired sequence, even one where the actuator valves are only accessed when they require translation.
It is another object of the present invention to provide a multiplexed hydraulic control system that allows the point in time that each actuator valve is serviced to be independent of all other actuator valves.
The above and other objects and advantages of the present invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.